“Distance images” are here not only to be understood as two-dimensional arrangements of distance picture elements, but also one-dimensional arrangements, also called profiles, i.e. distance images which each include a single row of distance picture elements disposed next to one another.
It is the object in distance measurement to measure the time between the transmission of pulsed electromagnetic radiation, here briefly called “signal” or “signal pulses”, and the arrival of the signal pulses reflected by targets as so-called “echo pulses”. These times can be converted into distances from the reflecting target due to the constant nature of the propagation speed of electromagnetic radiation. There are a number of embodiments of devices which satisfy this object which are here designated by the collective term “pulse TOF sensors”. TOF means “time of flight” and here designates the time which a signal pulse requires to the target and back.
Existing sensors generally (cf. also FIGS. 1 to 6) have                transmitters (15) with which the pulse of the electromagnetic radiation (14) is generated, such as a pulse laser (15) comprising a pulse laser diode (10) and a current pulser (11) for the generation of an optical pulse (14)        receivers (8) for the detection of the reflected signal pulses, such as receivers for light pulses comprising a photodiode or APD (avalanche photodiode) (2), a broadband amplifier (3) and a comparator (4) whose reference (5) is larger than 4.5 NEP of the noise of the analog output signal to reliably avoid the detection of noise pulses (“NEP” means “noise equivalent power” here—that power which corrects to the effective value of the noise),        a time measurement circuit with which the digitized, i.e. converted into a digital signal by means of a comparator, start pulses and echo pulses which belong together are converted into logical pulses or gate pulses which are either                    a. converted directly into voltage signals using TACs (time to analog converters), with this voltage subsequently being converted into a digital value using ADCs (analog to digital converters), or which            b. are used for the gating of an accurate measurement clock signal with a subsequent counting of the positive flanks of the measurement clock signal falling in the gate pulse, with the count representing the digital value of the TOF, or            c. which are used for the gating of the measurement clock signal with a subsequent counting of the positive flanks of the measurement clock signal falling in the gate pulse and are additionally used for the derivation of 2 TP pulses (TP=part period), with the TP pulse widths being converted into a digital value using TACs and a subsequent analog to digital conversion and the TOF being assembled from the part values, or            d. which are used for the gating of the measurement clock signal with a subsequent counting of the positive flanks of the measurement clock signal falling in the gate pulse and are additionally used for the derivation of 2 TP pulses, with the TP pulse widths being converted into a digital number using TDCs and the TOF being assembled from the part values.                        
The time measurement circuits or parts thereof are configured in a number of cases as an integrated “time measurement IC” due to the required switching speeds and complexity. Since a signal pulse can impact on a plurality of targets disposed sequentially so that a plurality of echo pulses enter into the time measurement circuit with different respective TOFs, their associated gate widths have to be measured. In some time measurement circuits, the occurrence of multiple echoes is taken into account in that at least the TOFs belonging to two echo pulses are measured using two time measurement circuits connected in parallel (double pulse evaluation).
These simple sensors have been extended to 2D sensors with the aid of a mirror scanner or to 3D sensors with the aid of two mirror scanners arranged perpendicular to one another, with 2D or 3D distance images then being able to be taken with them. Distance images are created in that a distance, and optionally a power, and not a color and a brightness as with images taken with CCD cameras, for example, is associated with each picture element (spatial angular segment of a larger spatial angle comprising an array of spatial angular segments).
Existing sensors are generally limited in their speed of taking the measured value by the maximum pulse repetition rate of the laser modules used. With pulse lasers having pulse laser diodes which have a power of some 10 watts required for the distance measurement of some 100 m, the pulse repetition rate is limited at around 20 to 30 kHz so that the taking of 2D or 3D distance images can only take place very slowly (with a few Hz) using these sensors. This is very slow in comparison with purely imaging sensors such as CCD cameras and is not suitable for the detection of dynamic processes within the spatial angle taken.
The sensitivity of existing pulse TOF sensors is generally >4.5 NEP of the noise due to the required avoidance of noise pulses.
An increase in the sensitivity above 4.5 NEP could generally be achieved in that the analog signal of the receiver was digitized by means of an ACD (analog-digital converter) with a high scanning rate. If the measurements are repeated and if the analog/digitally converted signals of the measurements are averaged, the noise would reduce by the square root of the averaging depth, whereas the signal pulses would be maintained at the same magnitude. The signal-to-noise ratio thereby improves proportionally to the square root of the averaging depth, which would generally correspond to an increase in sensitivity, provided a measuring time sufficient therefor would be available. However, the technical effort is enormous because the conversion rate has to be at some GHz and the computing effort for the averaging is necessarily proportional thereto. For this reason, sensors of this type have not achieved any importance in the market so that the sensitivity limit of today's pulse TOF sensors is generally larger than/equal to 4.5 NEP.
Disadvantages of the State of the Art of Today's Pulse TOF Sensors
It is therefore a disadvantage of the prior art that                the sensitivity (minimal signal-to-noise ratio) is limited to larger than/equal to 4.5 NEP since no noise pulses may be detected because they would be confusable with echo pulses and would falsify the result;        only one echo pulse, in rare cases two echo pulses, can be processed during the TOF and so reflections from rain, snow, fog or from the dirty termination glass of the sensor or from a plurality of impacted targets cannot be evaluated within a TOF or even make the measurement impossible, so that external applications for pulse TOF sensors can only be realized with great limitations;        no parallel processing of a plurality of signal pulses or parts of a signal pulse is possible;        no larger number of pulses such as noise pulses which generally occur increasedly at a low selection of the comparator threshold during the TOP can be measured with existing time measurement circuits;        no 2D distance image taking (profile) is possible without a scanner with movable mirrors;        a 3D distance image talking is only possible with two scanners with movable mirrors;        no sufficient speed for a 2D or a 3D distance image talking can be achieved.        
Furthermore, electronic cameras, predominately cameras with CCD chips as an areal sensor element, are known and have long been in use for digital image taking. These cameras are capable of supplying both static recordings (“images”) and continuous recordings (“moving images”) at high quality, in high resolution and at a sufficient speed and therefore have achieved an extraordinary product variety and product dissemination.
There are thus distance cameras in which a modified CCD chip is used for the distance measurement for each picture element in that a strong light pulse is transmitted and the reflected signal is integrated in the individual picture element sensors and the integration is interrupted at a suitable point in time so that a distance dependent charge amount is stored in each picture element sensor. Only very small ranges can be achieved with these sensors with a very large power and averaging effort so that they represent a poor compromise for all demands, with the exception of the lateral resolution.
It is desirable in many cases to obtain the information of distance and signal power in addition to the pure image information consisting of color and brightness per picture element. Such an “image”, which contains the information of distance and power per picture element, is called a distance image in the following. The type of sensors for which improved apparatus should be set forth in accordance with the invention in particular includes apparatus for continuous digital 2D and 3D distance image taking. In the following, a 2D distance image will also be called a “distance profile”.
The information of the distance images naturally consists of arrays of numbers. The position of the value in the array represents the angles of the distance picture element in a polar or spherical coordinate system so that the angles do not have to be indicated separately. In addition, the resolution and the type of the composition of the distance image must be known. A distance image can only be made visible for humans by stratagems such as the association of distances with colors. However, it takes considerably more effort to take the distance information as digital values from one or two images taken stereoscopically, and the accuracy worsens as the distance from the imaged objects increases. Since any controls, automation apparatus or measurement systems which process geometrical distance values require distances as digital values, there is a need for apparatus for the talking of distance images.
When taking distance images, in particular a restriction to the distance measurement by means of short pulses of electromagnetic radiation whose time of flight to a reflecting object and back is measured takes place in accordance with the invention. As already mentioned, this method is called “pulse TOF measurement”. This technique has the advantage that distances can be measured with it with a large working range and fast with an error which is dependent on the distance and low.
The simple sensors available today have been extended to 2D sensors with the aid of a mirror scanner or to 3D sensors with the aid of two mirror scanners arranged perpendicular to one another, with 2D or 3D distance images then generally being able to be taken with them. With existing sensors, the measuring speed is generally limited by the maximum pulse repetition rate of the laser modules used. Furthermore, the mechanical strain on the mirror scanners is very high because the total number of deflections per second is equal to the product of the column and line numbers and the repetition rate of the distance image. Generally, this is well above the mean repetition rate of the transmitters used so that existing sensors are not suitable for the taking of distance images for the named reasons alone.
It is problematic with the pulse TOF sensors known today that one or more of the following disadvantages and of the disadvantages already mentioned above is present in all sensors or measuring systems in existence today, namely that:                no sufficient speed for a 2D and 3D taking of distance images can be achieved, or at best only in very slow operation;        the mirror scanners are exposed to a high mechanical strain;        the suitability for outside applications is very limited.        
Furthermore, for the generation of distance images, there are                stereoscopic camera systems in which there is the possibility with two cameras whose optical axes have a base spacing to determine the distances in the picture elements by correlation. These systems suffer from a high calculation effort and from an error which increases considerably with the distance so that they only appear suitable in a very restricted application area;        triangulation systems in which a laser line is scanned over an object and is measured by means of a CCD camera. The distance profile can be calculated from the displacement of the lines. These systems have a low range and a measurement error which increases with the distance;        radar systems with which e.g. aircraft in space are measured for the purpose of air traffic control or tracking or ships are measured at sea for purposes of position tracking. These systems are, however, also only limited in their application to these cases due to the low radiation frequency used, which causes a low lateral resolution, and due to the long time of flight of the signals, so that practically no further applications have resulted.        
The present state of the art of the distance image generation can therefore be classified as insufficient.
In this connection, reference is made with respect to the prior art to the European patent application EP 1 522 870 (hereinafter KEM) which relates to a method and an apparatus for distance measurement. The invention makes at least partial use of the principle described in KEM. To avoid repetition, reference is herewith made to the disclosure content of the said EP 1 522 870 with respect to the KEM principle and the content of EP 1 522 870 is incorporated by reference in the present application.